Chromatin PTEN: Its Regulation And Function Phosphatase and tensin homolog (PTEN) functions as a major negative regulator of the PI3K signaling pathway. PTEN is frequently mutated in a variety of human malignancies including glioblastoma, prostate cancer, and breast cancer. Inherited PTEN mutations cause cancer-susceptibility conditions. PTEN is also known to have nuclear/chromatin functions, deregulation of which apparently causes chromosomal instability. However, the exact functions of chromatin PTEN and its molecular regulation remain poorly understood. Past research shows that proper subcellular localization of PTEN after genotoxic stress is regulated by molecular mechanisms that involve post-translational modifications. We recently demonstrated that chromatin PTEN significantly increases during mitosis, coinciding with an increase in PTEN phosphorylation in the C-terminal tail. Biochemical and molecular analyses revealed that Plk1 was responsible for PTEN phosphorylation on S380, a residue not targeted by any other known kinases. We and others have shown that PTEN specifically interacts with Cdh1 (APC/CCdh1) and WWP2, two ubiquitin E3 ligases. Chromatin PTEN removal during mitotic exit and the physical interaction between PTEN and Cdh1 was a proteasome-dependnent process. Furthermore, we observed that a cleaved form of WWP2 specifically is enriched during G2 and mitotic stages, correlating with chromatin PTEN accumulation. WWP2 silencing accelerates mitotic progression. We hypothesize that chromatin PTEN plays a crucial role in mitotic progression, whose subcellular localization and function are controlled by Plk1, Cdh1 and WWP2, and that its molecular deregulation leads to chromosomal instability and tumor development. To test the validity of our hypothesis, we will determine whether and how phosphorylation facilitates chromatin translocation of PTEN, dissect the role of PTEN ubiquitin E3 ligases in regulating its stability, and study the phosphatase-independent function of PTEN in supressing chromosomal instability and tumor development using both in vivo and in vitro models. Our proposed studies will not only elucidate the molecular mechanism by which chromatin PTEN is regulated during the cell cycle, but will also reveal how PTEN functions in maintaining chromosomal stability and suppressing malignant transformation. This line of research can lead to the identification of new molecular targets in the PTEN regulatory network that can be explored for cancer drug designs and development.